This invention relates to a CMOS radiation-measuring circuit with a variable threshold.
A complementary-symmetry MOSFET (CMOS) radiation-measuring circuit may include transistors configured to provide a digital output that changes from one state to another when the radiation dose absorbed by the circuit exceeds a threshold. The size of the devices used in the CMOS radiation-measuring circuit determines the radiation dosage that will cause the digital output of the CMOS radiation-measuring circuit to change from one digital state to another digital state. Hence, the design of a CMOS radiation-measuring circuit requires careful consideration of the size of transistors in the circuit.
When the voltage applied between the gate and source terminals of a MOSFET exceeds a certain voltage, the MOSFET turns on. This voltage is referred to as the threshold voltage. The threshold voltages of p-channel type MOSFET (PMOS) and n-channel type MOSFET (nMOS) devices are sensitive to ionizing radiation. Generally, the threshold voltage of an irradiated MOSFET device shifts due to an increase of trapped charge in the oxide and interface states.